1. Field
The present invention relates generally to data communication, and more specifically to techniques for determining transport format combinations (TFCs) supported for use in normal and compressed modes in a wireless (e.g., W-CDMA) communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication including voice and packet data services. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of system, including increased system capacity. A CDMA system is typically designed to conform to one or more standards, such as IS-95, cdma2000, and W-CDMA standards, all of which are known in the art and incorporated herein by reference.
The W-CDMA standard supports data transmission on one or more transport channels, and each transport channel may be associated with one or more transport formats (TFs) that may be used for the data transmission. Each transport format defines various processing parameters such as the transmission time interval (TTI) over which the transport format applies, the size of each transport block of data, the number of transport blocks within each TTI, the coding scheme to be used for the transport blocks in a given TTI, and so on. The use of multiple transport formats for a given transport channel allows different types or rates of data to be transmitted over the same transport channel. At any given moment, a specific transport format combination (TFC), which comprises one transport format for each transport channel, is selected from among a number of possible transport format combinations and used for all transport channels.
The W-CDMA standard also supports a xe2x80x9ccompressed modexe2x80x9d of operation on the uplink whereby data is transmitted from a terminal to a base station within a shortened time duration (i.e., compressed in time). The compressed mode is used in W-CDMA to allow a terminal in active communication with the system (i.e., on a traffic channel) to temporarily leave the system in order to perform measurements on a different frequency and/or a different Radio Access Technology (RAT) without losing data from the system. In the compressed mode for the uplink, data is transmitted by the terminal during only a portion of a (10 msec) frame so that the remaining portion of the frame (referred to as a transmission gap) may be used by the terminal to perform the measurements.
In accordance with the W-CDMA standard, the reduction in the transmission time for a compressed frame may be achieved by (1) reducing the amount of data to transmit in the frame, (2) increasing the coding rate, or (3) increasing the data rate. Reducing the amount of data to transmit in the compressed frame may be impractical for some applications, such as voice, since the data reduction may result in significantly reduced quality of service. Increasing the coding rate or data rate may be possible if the transmit power for the compressed frame is increased such that the energy-per-bit-to-total-noise-plus-interference ratio (Eb/Nt) for the compressed frame is similar to that for a non-compressed frame.
As noted above, a number of transport channels may be concurrently supported and a set of transport formats may be defined for each transport channel. A set of xe2x80x9cconfiguredxe2x80x9d transport format combinations may be defined for the transport channels, with each such transport format combination being associated with a particular relative transmit power level needed to achieve a target block error rate (BLER). The required transmit power for each transport format combination is dependent on (1) whether or not the terminal is in the compressed mode and (2) the parameter values defining the compressed transmissions in the compressed mode. To achieve high system performance, only the configured transport format combinations supported by the terminal""s maximum transmit power at the current channel conditions (i.e., those that can be transmitted with the required power for achieving the target block error rate) should be identified as those that may be selected for use. And only one specific transport format combination would then be selected from this set of supported transport format combinations for actual use at the next frame (shortest TTI) boundary.
There is therefore a need in the art for techniques for determining transport format combinations supported for use in normal and compressed modes in a W-CDMA system.
Aspects of the invention provide various techniques for determining valid (i.e., supported) TFCs from among all configured TFCs for normal and compressed modes. These techniques maintain sufficient historical information (in various forms) such that xe2x80x9cTFC qualificationxe2x80x9d may be accurately performed regardless of whether or not a TTI includes a compressed transmission. A number of TFC qualification schemes are provided herein. These schemes may be used in conjunction with an algorithm defined in W-CDMA whereby the determination of whether or not a TFC may be transmitted reliably is dependent on the TFC""s required transmit power for Y previous measurement periods and the maximum available transmit power at the terminal (described below). The information needed to determine whether or not a given TFC may be transmitted reliably comprises a Tx_power_requirement state for that TFC.
In a first scheme, a Tx_power_requirement state is maintained for each combination of compressed and non-compressed frames for each TFC. As used herein, xe2x80x9ccombinationxe2x80x9d refers to a specific combination of compressed and/or non-compressed frames for a given TFC and for a given TFC interval. The TFC interval is the longest TTI of any of the transport channels on which data is transmitted with this TFC. As used herein, xe2x80x9ctransport format combinationxe2x80x9d or xe2x80x9cTFCxe2x80x9d refers to a specific combination of transport formats that may be used for transmitting data on the configured transport channels. For each TFC selection interval, the specific combination applicable for the upcoming interval for each TFC is identified. The appropriate TFC state is then identified for each TFC based on this combination. (There is only one applicable combination for each TFC interval, and the states for all TFCs corresponding to this combination are determined.) The set of valid TFCs is finally determined based on whether they are in the proper state(s) (e.g., those in the Supported state and possibly the Excess-Power state defined in W-CDMA).
In a second scheme, two Tx_power_requirement states are maintained for each TFC for the normal and compressed modes, i.e., one state for the normal mode (which has no transmission gaps) and the other state for the combination requiring the most transmit power (e.g., the worst possible case, or worst case based on the configured transmission gap pattern sequences). For each TFC selection interval, the applicable combination is identified for each TFC, and the valid TFCs are then determined based on whether or not they are in the proper state(s).
In a third TFC qualification scheme, a single Tx_power_requirement state is maintained for each TFC for both normal and compressed modes. This single Tx_power_requirement state may be maintained for each TFC for a compressed mode relative power requirement, xcex1cm,i, which may be defined as the relative power requirement for the normal mode, xcex1ref,i, times an offset xcex1offset,i (i.e., xcex1cm,i=xcex1ref,ixc2x7xcex1offset,i).
In a fourth scheme, a number of Tx_power_requirement states is maintained for a set of xe2x80x9cbinsxe2x80x9d that cover the total range of relative required transmit powers for all TFCs for the normal and compressed modes. Each combination for each TFC is associated with a particular relative required transmit power, and may therefore be associated with a specific bin and further utilize the Tx_power_requirement state maintained for that bin.
In a fifth scheme, a set of relative power requirement xe2x80x9cthresholdsxe2x80x9d are determined and maintained for Y measurement periods. The relative power requirement threshold, xcex1th(k), for each measurement period may be defined as the ratio of the maximum available transmit power, Pmax, over the required transmit power for a reference transmission, Pref(k) (i.e., xcex1th(k)=Pmax/Pref(k)). The state of each TFC may then be determined based on the TFC""s relative required transmit power for the upcoming interval, the set of relative power requirement thresholds, and a (e.g., 2-bit) state and a timer maintained for each combination for each TFC.
These various schemes and their variants and various other aspects and embodiments of the invention are described in further detail below. The invention further provides methods, program codes, digital signal processors, receiver units, terminals, base stations, systems, and other apparatuses and elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.